The only Union to join is the one at your depot, as they are normally the only one recognised by the company and they should give the Reps time to be present at any disciplinary meetings. Also good Reps will know terms & conditions etc.
Some bigger depots may have different unions for Drivers (Unite/URTU) and warehouse workers (USDAW).
Also it may be to late, as many union rules state subscriptions need to be paid before commencement of any disciplinary action.
Think of joining a Union as like having RAC/AA breakdown cover, you pay it in the hope you don’t need to use it, but do so as peace of mind.